The invention relates to optical fibers of the graded index type consisting of a silicate glass containing magnesium oxide together having one or more metal oxides with cations which can be exchanged for the magnesium cation. The refractive index gradient is caused by a decreasing metal oxide concentration and an increasing magnesium oxide concentration from the axis radially outward in the direction of the fiber circumference.
Optical fibers of this type are known from published United Kingdom Patent Application GB No. 2,002,341. The glasses described therein are of the boro-silicate type and contain alkali metal oxides in addition to metal oxides having exchangeable cations, such as alkaline earth oxides. The optical fibers are produced by means of the double crucible technique.
Purer starting materials are becoming available to a greater extent for producing glasses for the manufacture of optical fibers by the double crucible method. As a consequence, the attenuation of the light signal through such a fiber depends to a lesser extent on impurities in the glass originating from the starting materials.
Attenuation of the light signal, however, is also due, in part to so-called intrinsic attenuation, which depends on the properties of the glass constituents per se.
The wavelength range between approximately 1100 and 1500 nm is becoming of increasing interest in optical telecommunications.
It has been found that at wavelengths longer than approximately 1200 nm boron oxide absorbs light in a troublesome manner. Because of this high intrinsic absorption, borosilicate glasses are therefore not suitable for use in this wavelength range if low intrinsic attenuation is desired.